Optical navigation devices use a light source to illuminate a navigation surface so that an optical imager can generate digital images for computing motion. For example, an optical imager can image a finger on a surface and generate a navigation signal based on comparisons of sequential images. However, as the size of conventional navigation devices is reduced, the physical space for the optical elements and sensors for detecting the optical navigation input are also reduced.
For example, some conventional optical navigation device packages have a thickness, or optical height, of about 2.5 mm. The optical height refers to the distance from an optical finger interface surface to a corresponding sensor. The optical height is also referred to as the optical track. An optical height, or thickness, of 2.5 mm is considered too thick for some implementations of handheld devices such as cellular telephones and small portable personal computing (PC) peripherals.
In conventional optical navigation devices which use optical lenses to direct the light from the optical finger interface surface to the corresponding sensor, it may be impractical to reduce the optical height below about 2.5 mm. In attempting to reduce the optical height of conventional optical navigation devices, the lower total optical height makes it difficult to properly mold the imaging lens. This also has the adverse effect of reducing the accuracy of the imaging lens. Tolerances for the optical navigation device also become very tight, and designing and producing an imaging lens according to such tight tolerances increases the cost for the package, due to the complexity of the design.